I would tell Mr. Homeowner I cannot ethically perform the task he is requesting. I cannot replace the existing markers without verifying that they are indeed original undisturbed monuments marking the corners of his property. I would further suggest that no other licensed surveyor can ethically perform the task either.
In response to his "threat" to do it himself, I would suggest he read Section 716.6, subsection 1 of the Code of Iowa and, perhaps, consult an attorney before proceeding:
Section 716.6, subsection 1
a. Criminal mischief is criminal mischief in the fourth degree if the any of the following apply:
(1) The cost of replacing, repairing, or restoring the property so damaged, defaced, altered, or destroyed exceeds two hundred dollars, but does not exceed five hundred dollars.
(2) The person intentionally injures, destroys, disturbs, or removes any marker placed on any tract of land, street, or highway, designating any point, course, or line on the boundary of the tract of land, street, or highway, if the marker was placed at such location by a land surveyor licensed under chapter 542B, or by any person directed by a licensed land surveyor. A governmental entity and employees of such an entity are exempt from prosecution under this subparagraph for projects performed pursuant to section 314.8. A licensed land surveyor and persons under the direction of a licensed land surveyor are also exempt from prosecution under this subparagraph for removing an existing marker in order to place an upgraded or more suitable marker in the same location.
(3) The person intentionally injures, destroys, disturbs, or removes any monument that has been established by the national geodetic survey, Iowa geodetic survey, or any county geographic information system for use in the determination of spatial location relative to the specified Iowa state plane coordinate system or precise elevation datum. A governmental entity and employees of such an entity are exempt from prosecution under this subparagraph for projects performed pursuant to section 314.8.
b. Criminal mischief in the fourth degree is a serious misdemeanor.
I would further explain that, in Iowa, a serious misdemeanor is punishable by a fine up to $1875 and up to a year in jail.